I think the biggest difference in how we're looking at this is that I'm using the calculators to examine the change in performance from stock to F-L GTR, while you're looking at absolute numbers. Using my approach, the change in 1/4 mile trap speed from 107 to 113 requires about +50 RWHP, while using your approach, the observed 113 mph is consistent with 350 RWHP. I prefer the incremental approach, as it minimizes the influence of factors that consistently skew the results for both the stock and F-L cars (such as the excellent CS aerodynamics that increase its trap speed compared to, say, an Impala).

Regarding the F-L HP claim, I think it would be +86 HP if the 3.8 X51 were stock, but they add their own custom exhaust, and it was my recollection that F-L was claiming something like 390-400 HP.

In any case, a good discussion. I'm signing off on the issue of the actual HP of 3.8 X51 installations, so this thread can get back on the topic of the TPC turbo cars.